Method of exploring a region below a surface of the earth

ABSTRACT

A passive method for exploring a region below the surface of the earth. The method comprises using a plurality of sensors to obtain seismic data obtained by recording ambient seismic interface waves in a frequency range whose lower limit is greater than 0 Hz, and whose upper limit is less than or equal to substantially 1 Hz. The sensors may be sensitive to three orthogonal components. Recordings may be made simultaneously by all sensors and normalization of data is unnecessary. The sensors may be moved and clean data may be selected. Local dispersion curves may be determined to improve vertical resolution. The data are processed so as to obtain a measure of the energy in a frequency band within the frequency range. The energy measure may be calculated by integrating the spectrum in the frequency domain over a desired frequency range. The resulting calculated energy provides information about the region of the earth being explored.

PRIORITY CLAIM

The present application is a National Phase entry of PCT Application No.PCT/GB2008/051223, filed Dec. 22, 2008, which claims priority from GreatBritain Application No. 0724847.9, filed Dec. 20, 2007, the disclosuresof which are hereby incorporated by reference herein in their entirety.

TECHNICAL FIELD

The present invention relates to a method of exploring a region below asurface of the Earth. The present invention also relates to an apparatusfor performing such a method, a program for performing data processingof such a method, a computer programmed by such a program, acomputer-readable medium containing such a program and transmissionacross a network of such a program. Such a method may be used, forexample, to search for new sources of hydrocarbons and to monitor knownhydrocarbon reserves.

BACKGROUND ART

There are various known techniques for exploring the structure and/orcomposition of regions below the surface of the earth, including belowthe sea bed. Some of these techniques are “active” in that they requirethe use of a specific controlled source of energy to probe the regionand then measure the effect of the region on the transmitted energy. Atypical example of such an active technique is the well-known seismicexploration technique, in which one or more sources produce impulsive orswept-frequency pressure waves which penetrate the earth and thereturned pressure variations are measured by geophones or hydrophones.

Other techniques are of the “passive” type in that they do not requireenergy to be produced by a source but, instead, make use ofalready-existing energy to provide information about the structure orcomposition below the surface of the earth. One such technique is knownas hydrocarbon microtremor analysis (Hymas), for example as disclosed inDangel et al, 2001, “Phenomenology of tremor-like signals observed overhydrocarbon reservoirs (just related and higher frequencies)”. Hymasattempts to measure a continuous signal or tremor in a frequency rangebetween 1 and 10 Hz over the reservoir (not “outside” or “beside” it).As illustrated in FIG. 1 of the accompanying drawings, sensors such as 1and 2 embedded in the surface 3 of the earth measure such tremors. Thesensors 1, 2 are in the form of extremely sensitive seismometers havinga sufficiently broad bandwidth to be sensitive to the frequency range ofinterest. The seismometers are typically arranged as a two-dimensionalarray or grid with the spacing between seismometers typically being froma few hundred meters to a kilometer, depending on the spatial resolutionrequired. The outputs of the seismometers are recorded for a period oftypically 24 hours. It is not necessary for the recordings to be madesimultaneously and fewer seismometers may be used by changing theirlocations after each recording period.

This technique is based on the assumption that a hydrocarbon reservoir 4interacts with ambient seismic waves to modify the waves in a measurableway. FIGS. 2 and 3 of the accompanying drawings illustrate this. Theseismometer 1 is located such that there is no hydrocarbon reservoir inthe region of the earth below it. FIG. 2 illustrates the result ofanalysing the recorded data from the sensor over a recording period oftypically 24 hours. The resulting data are plotted as a frequencyspectrum of amplitude in arbitrary units against frequency in Hertz(Hz). The seismometer 2 is located above the hydrocarbon reservoir 4 andthe spectrum obtained from the recorded data is shown in FIG. 3. It isbelieved that the presence of significantly increased energy in thefrequency band around 3 Hz results from the modified seismic waves 5produced by the hydrocarbon reservoir 4 and so provides a directindication of the presence of the hydrocarbon reservoir.

A problem with the Hymas technique is that it is based on the detectionand recording of very weak seismic tremor signals. However, humanactivity provides relatively high amplitude noise in the frequency rangeinvestigated in the Hymas technique. For example, noise resulting fromvehicles, pumps and drilling occurs in this frequency range and may makethe Hymas technique unusable in many situations.

Another known passive technique is referred to as ambient noisetomography (ANT). This technique is based on the use of conventionalseismometers which have been recording very low frequency ambient noisefor very long periods of time, for example of the order of years. Thefrequency range of interest is below 0.5 Hz and mainly below 0.05 Hz.Although the seismometers may be spaced apart by hundreds of meters forspecial cases, they are typically spaced apart by hundreds of kilometersand even globally around the earth. It is necessary for recordings to bemade simultaneously and this reveals subsurface information between thelocations of pairs of seismometers. An example of this technique isdisclosed in Bensen et al 2007, “Processing seismic ambient noise datato obtain reliable broad-band surface wave dispersion measurements”.

The data from such seismometer arrangements are processed so as toextract the Green Function by cross-correlation of two simultaneousrecordings. The results for each frequency are then used subjected to adispersion analysis followed by a tomographic inversion so as to obtaina laminar image. The resolution is such as to provide structuralinformation about the earth on the continental scale. Thus, thistechnique is generally not of particular interest when looking atsmaller regions, for example, when exploring for or monitoringhydrocarbons, because of the insufficient spatial resolution.

Russian Patent no. 2271554C1 discloses a technique for recording andanalyzing ambient noise in a frequency range below 15 Hz. In a firstphase of this technique, ambient noise is simultaneously recorded(“synchronised”) by means of at least three sensors which are sensitiveto the vertical component of the ambient noise signal. The data are thenanalyzed to estimate dispersion curves within the area beinginvestigated.

In the next phase, ambient noise is recorded using at least twosynchronised sensors sensitive to the vertical component. One of thesensors is used as a permanent station at the center of the area beingsurveyed during the whole of the survey. The or each other sensor ismoved to different locations within the area being surveyed so as torecord the vertical component of the ambient noise at each of thelocations for an unspecified time period. The recordings obtained by thenon-fixed sensors are then calibrated by calculating the differencesbetween the power spectra of the permanently located sensor and the“moving” sensors. From the estimated dispersion curve, a depth iscalculated, in an unspecified way, for each frequency. Maps are thendrawn for each frequency or depth of the power spectra differences.Because the sensors are sensitive only to the vertical component of themeasured ambient noise, this technique is limited to Rayleigh waves.

SUMMARY OF THE INVENTION

According to a first embodiment of the invention, there is provided amethod of exploring a region below a surface of the Earth, comprising:using a plurality of sensors at a plurality of locations to obtainseismic data by recording ambient seismic interface waves, each sensorbeing sensitive to three substantially orthogonal components of thewaves in a frequency range whose lower limit is greater than or equal to0 Hz and whose upper limit is less than or equal to substantially 1 Hz;and processing the data to obtain a measure of the energy in a frequencyband within the frequency range.

Each of the sensors may be sensitive to two orthogonal horizontalcomponents and one vertical component.

The method may comprise rotating the components so that one thereof isaligned in a desired direction. The desired direction may be a directionof arrival of the waves.

According to a second embodiment of the invention, there is provided amethod of exploring a region below a surface of the Earth, comprising;using a plurality of sensors, each of which is located in turn at aplurality of locations, to obtain seismic data by recording ambientseismic interface waves in a frequency range whose lower limit isgreater than or equal to 0 Hz and whose upper limit is less than orequal to substantially 1 Hz; and processing the data to obtain a measureof the energy in a frequency band within the frequency range.

The method may comprise locating the sensors at a plurality of differentcombinations of the locations in turn and making at least partiallysimultaneous recordings at each of the combinations. The plurality ofsensors may comprise first and second sensors and the first and secondsensors may be moved alternately to provide the different combinations.The method may comprise normalizing the data provided by each of thefirst and second sensors following a change of position thereof withrespect to the data provided by the other of the first and secondsensors.

According to a third embodiment of the invention, there is provided amethod of exploring a region below a surface of the Earth, comprising:using a plurality of sensors at a plurality of locations to obtainseismic data by recording ambient seismic interface waves in a frequencyrange whose lower limit is greater than or equal to 0 Hz and whose upperlimit is less than or equal to substantially 1 Hz; selecting data in atleast one time sub-interval of each of at least some of the recordingsin accordance with the quality of the data in the sub-interval; andprocessing the selected data to obtain a measure of the energy in afrequency band within the frequency range.

The data may be selected in accordance with the energy contained in thesub-interval. The data may be selected from the sub-interval of eachrecording containing a smallest amount of energy.

According to a fourth embodiment of the invention, there is provided amethod of exploring a region below a surface of the Earth, comprising:using a plurality of sensors at a plurality of locations to obtainsimultaneously recorded seismic data by recording ambient seismicinterface waves in a frequency range whose lower limit is greater thanor equal to 0 Hz and whose upper limit is less than or equal tosubstantially 1 Hz; and processing without normalising the data toobtain a measure of the energy in a frequency band within the frequencyrange.

The method may comprise a combination of at least two of the methodsaccording to the first to fourth embodiment of the invention.

The lower limit may be greater than or equal to one of 0.005 Hz, 0.001Hz, 0.01 Hz and 0.1 Hz.

The data may be recorded at each location over a predetermined period.The period may be greater than substantially half an hour.

The method may comprise processing the data to obtain the measure of theenergy of at least one of Rayleigh, Love and Scholte waves.

The locations may be spaced apart by at least substantially 100 meters.The locations may be spaced apart by less than substantially fivekilometers. The locations may be arranged as a two-dimensional array.

The seismic interface waves may be recorded at or adjacent an interface.

The data may be processed to obtain measures of the energy in aplurality of different frequency bands within the frequency range. Thefrequency bands may be contiguous or non-overlapping.

The or each frequency band may have a width between 0.001 Hz and 1 Hz.

The frequency band or at least one of the frequency bands may comprise adiscrete frequency.

The method may comprise deriving the or each measure of the energy fromthe amplitudes of the recorded waves in the or the respective frequencyband.

The method may comprise deriving the or each measure of the energy fromthe number of the recorded waves in the or the respective frequencyband.

The method may comprise converting the data to the frequency domain. Themethod may comprise deriving the or each measure of the energy byintegrating the converted data over the or the respective frequencyband.

The method may comprise normalizing the amplitude of the data. Themethod may comprise converting the amplitude of the data to one of firstand second values according to the sign of the amplitude.

The method may comprise filtering the data to attenuate or remove atleast some frequencies outside the or each band.

The method may comprise providing a visualization of the or eachmeasure. The measures may be represented as a map of the region.

According to another embodiment of the invention, there is provided anapparatus arranged to perform a method according to the first embodimentof the invention.

According to another embodiment of the invention, there is provided acomputer program arranged to control a computer to perform processing ina method according to the first embodiment of the invention.

According to another embodiment of the invention, there is provided acomputer containing or programmed by a program according to the thirdembodiment of the invention.

According to another embodiment of the invention, there is provided acomputer-readable medium containing a program according to the thirdembodiment of the invention.

According to another embodiment of the invention, there is providedtransmission across a network of a program according to the thirdembodiment of the invention.

It is thus possible to provide a technique which may be used to explorea subsurface of the earth in a passive or non-intrusive way. Thistechnique is applicable to a very wide range of locations includingonshore and offshore. It may even be possible to use this technique inarctic regions. Further, this technique may be used in protectednational parks and the like where it is not possible or not permitted touse active seismic sources.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further described, by way of example, withreference to the accompanying drawings, in which;

FIG. 1 is a diagrammatic sectional view of a region of the earthillustrating the Hymas technique;

FIGS. 2 and 3 are graphs of amplitude against frequency illustrating thespectra of signals obtained from the Hymas technique of FIG. 1;

FIG. 4 is a cross-sectional diagram of part of the Earth illustratingdata acquisition;

FIG. 5 is a block diagram of a computer for performing processing ofacquired data;

FIG. 6 is a flow diagram illustrating a method constituting anembodiment of the invention;

FIG. 7 illustrates the effect of a frequency filtering step of themethod of FIG. 6;

FIG. 8 is waveform diagram illustrating the effect of an amplitudenormalization step of the method of FIG. 6;

FIG. 9 is a diagram showing particle motion for a Rayleigh wave; and

FIG. 10 is a graph of depth in wavelengths against relative displacementin arbitrary units for the vertical and horizontal components ofRayleigh waves.

DETAILED DESCRIPTION

The present technique records and analyzes ambient seismic noise. Suchnoise includes all natural signals (main energy below ˜0.5 Hz) and alluncontrolled man-made or “anthropogenic” sources (main energy above ˜0.5Hz). Except for some transient seismic signals (e.g. earthquakes,explosions and so on), these ambient noise records look like incoherentmotion of the subsurface. These incoherent signals are waves ofdifferent types and surface waves are mainly expected within theserecordings due to their characteristic properties.

The present technique mainly records and analyzes seismic interfacewaves (SIW), such as Rayleigh, Love, Scholte waves. In contrast to bodywaves (longitudinal and shear waves, or in seismology P- and S-waves),which are normally used for hydrocarbon exploration, these SIW propagatealong an interface (e.g. the surface or sea bottom). These SIW also haveamplitudes and energy below the surface, which are partly higher thantheir amplitudes directly at the interface. The depth of the highestamplitude depends on the frequency and the propagation velocity of therespective SIW. As a rough rule of thumb, the penetration of SIWincreases with decreasing frequency. Active sources usually used forhydrocarbon exploration have their main frequency above 10 Hz (frequencyband 1). In this frequency range, the penetration of useable amplitudesof SIW will be less than 10 meters (m). Seismologists are using SIW withfrequencies far under 0.1 Hz (frequency band 3) to obtain information onthe earth's crust and upper mantle. The present technique uses wavesbetween these two frequency bands (band 2) to derive the informationabout the subsurface between 0 m and 8000 m depth.

The active excitation of SIW in frequency band 1 is possible withconventional seismic sources (explosion, vibration and so on) or byhuman noise (e.g. cars, trucks, industry). In band 3, seismologists areusing SIW excited by earthquakes. The active excitation of SIW infrequency band 2 is not really possible (except by nuclear explosions).The present technique thus makes use of ambient noise.

In frequency band 2, the SIW are mainly excited by waves and storms onthe ocean. It is possible to measure these SIW everywhere in the world(for example as disclosed by Webb 1998: “Broadband seismology and noiseunder the ocean”, in Reviews of Geophysics) and they are usually called“microseisms”.

In general, the analysis of amplitudes of recorded seismic signals/wavesis very difficult, for example due to the big influence of theindividual response function (including the coupling to the earth) ofeach seismometer. However, these amplitudes contain further oradditional information compared with the dispersion of SIW which isnormally analyzed by dispersion analysis.

FIG. 4 illustrates a first region of the Earth, indicated generally at10, and a second region of the Earth, indicated generally at 11,providing examples of regions which may be explored using the presenttechnique. The region 10 is located offshore below the sea bed 12whereas the region 11 is located onshore below the land surface 13. Thesea or ocean is shown at 14 with waves 15 propagating along the surfaceand breaking on a shore 16.

FIG. 4 illustrates two techniques for recording seismic interface wavesin order to explore the two regions 10 and 11. In order to explore theregion 10, a plurality of broadband seismometers 20 of sufficientsensitivity is disposed on the sea bed 12 at a plurality of locations,for example arranged as a grid or array above the region 10. Thelocations of the seismometers 20 may, for example, be chosen to be atwo-dimensional array covering the region 10 and with the spacingbetween adjacent locations being from approximately 100 meters toapproximately 5 kilometers. In particular, the spacing determines thespatial resolution achievable by the present technique as eachseismometer provides data relating to the part of the region generallybelow the location. Typically a square or rectangular grid of locationsis chosen. The seismometers may be sensitive to the vertical componentof the seismic interface wave or may also be sensitive to otherhorizontal components.

The seismometers are selected so as to provide sufficient sensitivity tomeasure the seismic interface waves in the frequency range of interest,which is below substantially 1 Hz. The lower limit of the range may bechosen according to the depth of the region 10 to be investigated.Frequencies down to 0 Hz may be recorded but a lower limit which maywell be acceptable in most situations is thought to be 0.005 Hz. Theseismometers must therefore provide sufficient sensitivity throughoutthis frequency range.

In order to explore the region 11, onshore seismometers 21 are arrangedas a grid in a similar way.

The regions 10 and 11 are intended to depict structures illustrated at22 comprising known hydrocarbon reservoirs or prospects for suchreservoirs. The present technique may be used, either alone or incombination with other subsurface exploration techniques, to look fornew hydrocarbon reservoirs or to monitor existing reservoirs, forexample during production or reservoir emptying.

The seismic interface waves are recorded for a predetermined period ateach of the locations of the grid. It is believed that a recording timeat each location of a few hours will generally be sufficient and it isexpected that most locations will allow a recording time of less than 24hours. It is believed that a recording time of the order of half an hourwill be sufficient in some situations.

Although it is possible for the seismic interface waves recorded by theseismometers to be processed in real time during recording, it is morelikely that the recordings will be stored for subsequent processing. Theseismic data from the seismometers are therefore typically converted tothe digital domain and stored on any suitable storage medium. The dataare subsequently retrieved when convenient and processed so as toanalyze the regions 10, 11.

The recorded data are processed by an apparatus in the form of acomputer, an example of which is illustrated in FIG. 5. The computercomprises a central processing unit (CPU) 30 controlled by a programstored in a read-only memory (ROM) 31. A read/write memory in the formof a random access memory (RAM) 32 is provided for temporary storage ofdata during processing and, if appropriate, for non-volatile storage ofsuch data, for example including the results of processing. An inputinterface 33 is provided to allow the data recorded from theseismometers to be entered into the computer and an output interface 34is provided for supplying the results of the processing. For example,the output interface 34 may provide means for visualising the results ofprocessing.

FIG. 6 is a flow diagram illustrating the processing performed by thecomputer shown in FIG. 5. The recorded data are provided at 40, forexample by loading the recorded data into the input interface 33. Thedata are supplied to a filtering step 41, which provides optionalfiltering of the data. For example, the step 41 may apply a highpassfrequency filter with a suitable cut-off frequency, for example of 0.01Hz, in order to eliminate or attenuate energy outside the frequencyrange of interest. Such filtering may be used to eliminate or attenuatestrong low frequency amplitude variations originating outside the region10, 11 of interest. FIG. 7 illustrates at 35 the “raw” data recorded byone of the seismometers over approximately a 24 hour period. The curve36 illustrates the result of filtering.

It is also possible to provide bandpass frequency filtering in the step41 so as to attenuate or eliminate frequencies outside the frequencyrange of interest. It is further possible to use a bandpass filter toselect a band of frequencies or a discrete frequency of particularinterest. The filtering step 41 may provide simultaneous or sequentialfiltering into different frequency bands or discrete frequencies withthe subsequent processing being repeated for each frequency band ordiscrete frequency, for example so as to provide information about thestructure and/or composition at different depths within the region 10,11 as described hereinafter.

The optionally filtered data are then supplied to a normalising step 42which, again, is optional and which may be used to provide amplitudenormalization of the recorded data. Such normalization may improve thedata quality, for example to reduce the effects of amplitude variationsresulting from different recording times or from specific artifacts,such as earthquakes.

Various normalization techniques are possible and may be used accordingto the requirements and according to the quality of the recorded data.For example, in order to remove “strong events” such as earthquakes,amplitude limiting may be applied so as to limit the positive andnegative amplitudes to desired thresholds. Any desired degree oflimiting may be applied, for example to limit all of the positive andnegative peaks in the recorded data to predetermined thresholds. Suchnormalization may be taken to the limit in the form of “one bitnormalization”. In such normalization, only the amplitude sign is takeninto account so that each peak is limited to either a common positivevalue or a common negative value according to the sign of the originalamplitude. The result of such normalization is illustrated in FIG. 8.

Other, non-limiting, amplitude normalization may be performed. Forexample, absolute mean normalization may be used.

The time-domain data are then supplied to a step 43, where they areconverted to the frequency domain so as to provideamplitude-against-frequency spectrum information. Any suitabletransformation may be used and a typical example of such atransformation is a fast Fourier transform. The data in the frequencydomain are then supplied to a step 44 for calculating the energy in allor part of the frequency range covered by the data. For example, theamplitude value for one or more specific discrete frequencies may bedetermined in the step 44. Alternatively or additionally, the step 44may integrate the amplitude information in the data from the step 43over one or more frequency bands or windows. For example, the energy ata set of discrete frequencies or found by integrating over a set ofcontiguous or non-overlapping frequency windows may be calculated by thestep 44. As described hereinafter, it is believed that the energy atdifferent frequencies or in different frequency bands providesinformation more directly related to different depths within the region10, 11.

The result of the step 44 is the calculation of the energy at a specificfrequency or in a specific frequency band at the location of theseismometer which recorded that part of the data. This may represent theamplitude of the seismic interface wave at a discrete frequency or in afrequency window. In the case where one bit normalization has beenperformed in the step 42, the calculation indicates how often a wave ofthe specific frequency or of a frequency within the specific band occurswithin the recorded data at the location of the seismometer. Both ofthese “measures” therefore provide a measure of the energy in theseismic interface wave at that frequency or in that band at the locationof the seismometer.

The calculated measure of energy may be used directly in order toprovide information about the region 10, 11 below the locations 20, 21.However, the results of the step 44 may alternatively or additionally besupplied to an optional visualizing step 45 for providing a visualoutput, for example in the form of an image on a display and/or aprinted hardcopy. This may assist in interpreting the results of theexploration. For example, the values calculated at the same frequency orin the same frequency range for each of the seismometer locations may bedisplayed as a two-dimensional horizontal map with the individual valuesbeing represented at the locations of the seismometers in the horizontalplane.

It is also possible to provide a two-dimensional representation ofamplitude plotted against one-dimensional position, for example on thehorizontal axis, and frequency or frequency band, for example on thevertical axis. By combining a plurality of such “sectional views” in theorthogonal horizontal spatial direction, a three-dimensionalvisualisation may be provided. An inversion may be performed to convertfrequency to spatial depth so as to provide a two-dimensional orthree-dimensional visualisation of the structure or composition of theregion 10, 11 below the grid of seismometer locations.

Many visualization tools exist for providing visual representations ofsubsurface regions. Any suitable tool may be used in the step 45 inorder to provide any desired visualization based on the energycalculations performed in the step 44.

Any type or types of surface interface waves may be recorded in order toperform the present technique. Examples of such waves are Rayleighwaves, Love waves and Scholte waves.

FIG. 9 illustrates the motion of a medium 50 on whose surface 51 aRayleigh wave is propagating. Cartesian axes are illustrated at X, Y andZ with the wave propagating in the horizontal direction X and the Z axispointing downwardly into the depth of the medium 50.

The particle motion below the surface when a Rayleigh wave ispropagating along the surface 51 is illustrated at 52. Thus, the surfaceor interface Rayleigh wave is propagated by means of particle motion notjust at the surface 51 but also within the medium 50 and each particlerepeatedly describes a closed loop.

FIG. 10 illustrates depth in wavelengths against relative maximumdisplacement of particles for a Rayleigh wave and for different valuesof v. This illustrates that the maximum displacement or amplitude in thevertical direction does not occur at the surface 51 or other interfacebut instead occurs at a depth of approximately 0.2 wavelengths below thesurface.

Although the mechanism underlying the present technique is not fullyunderstood at present, it is believed that the structure and/orcomposition of the subsurface region of the earth interacts withRayleigh or other seismic interface waves in a way which is encoded inthe amplitude of such waves at the interface. Further, it is believedthat the measurable effect may be largely a function of the maximumparticle displacement or amplitude of the medium through which the waveis propagating. Although all depths may have an effect on the wave asmeasured at the surface, the maximum effect for the vertical componentof Rayleigh waves, for example, may largely result from the structure orcomposition of the earth about 0.2 wavelengths below the surface orother interface. Thus, it is believed that the amplitudes of seismicinterface waves such as Rayleigh waves measured at the surface and ofdifferent frequencies principally provide information about thestructure or composition at different depths below the measurementlocations.

It is not yet known which mechanism is affecting the seismic interfacewaves. However, it is currently believed that one or more of thefollowing mechanisms may be responsible for affecting the measuredamplitudes:

-   -   Scattering of the surface interface waves;    -   Different attenuations of the surface interface waves;    -   The effects of different wave velocities;    -   The effects of different fluids.

Thus, the amplitudes measured at the interface may relate to thestructure below the measurement location, to the nature of the materialsuch as whether the material is solid, water or hydrocarbon, or both.

The following describes various examples of the present technique. Forthe data acquisition, at least two sensors are needed which aresensitive to at least one component. Sensors may be used which aresensitive to three components (for example two horizontal and onevertical). To investigate the Rayleigh wave part of the ambient noise,the vertical component or recording is analyzed. To investigate also theLove wave part and the radial part of the Rayleigh wave, it is necessaryto know the main direction of the incident surface waves and a simplerotation of the data to this direction may be applied. This direction ismostly related to storm events in the vicinity (a few hundred orthousand kilometers away). Meteorological observations may provide thisinformation. It is also possible to use the free available data ofseismological stations to locate these storms or to identify the maindirection. If at least three sensors are used for data acquisition, itmay be possible to perform an array analysis, which may be of knowntype, to resolve the main direction of incident waves.

The sensors are placed at several locations within the surveyed area,for example at least two to see differences between these two locations.The locations for measurements within the surveyed area are preferablyon an equidistant grid or otherwise evenly distributed. This is notessential and useful results can also be obtained by irregular surveylayouts, which might be chosen e.g. due to topography or priorsubsurface information.

There are two possibilities for covering a region to be explored. 1.There are enough sensors available to record with all sensorssynchronously so that every measurement location will have one sensor.In this case, no normalization of the data is necessary. This way opensthe possibility of combining this method, for example, with ambientnoise surface wave tomography (ANSWT) or other array method. 2. Thereare not enough sensors available so that at least some of the sensorshave to be moved to other locations. In this case, 2a a permanentstation may be used to normalize the data afterwards. The permanentstation may be placed within the surveyed area or in the vicinity of thesurveyed area. It is also possible to use publicly available data ofseismological observatories in the vicinity (the distance depends on thearea but may, for example, be a few 100 km) of the surveyed area. 2b. Ifthere are less sensors available (at least two), it is also possible towork without a permanent station. In this case, for example, a“temporarily permanent” station may be used. For example, with twosensors, data acquisition starts with the sensors at locations A and B.After the first recording time, the second sensor is moved from B to D.Afterwards the first sensor is moved from A to C and so on. By thismethod, it is possible to cover the entire area. Normalization isdescribed within the processing description hereinafter.

The recording time for each seismometer is typically a few hours orless. This depends also on the desired penetration. Higher frequenciesmean less penetration and less recording length. It is preferable for atleast part of the recording time to be substantially free from locallygenerated transient noise.

Another possibility is to record for longer (˜days) to measure the fullvariation of ambient noise amplitudes over time. In this case, thequietest time periods may be selected to compare only the spectra ofthese time intervals. It is assumed that, after each high noise period,there is a calm period which is comparable in amplitude with theprevious one.

After the data acquisition is finished, seismometer records areavailable of different locations over the surveyed area. For eachseismometer record, the following steps are applied. Not all aremandatory.

-   -   1. Filtering    -   2. Calculation of FFT/spectrograms    -   3. Selection of time periods    -   4. Normalisation    -   5. Value calculation

In the following, each step is described in more detail. If there arealternatives in the way of processing the data, this is noted. Thefollowing steps can be applied to all three components.

1. Filtering: A frequency filter is applied to focus on a specificfrequency band. This could be:

-   -   a high pass frequency filter with a cut off frequency of around        0.01 Hz to eliminate strong low frequency amplitude variations;    -   a band pass filter for a specific frequency band. If a value for        a discrete frequency only is to be calculated (see point 4),        than a band pass filter may be applied for this frequency:    -   to obtain values for many frequencies, the processing flow may        be repeated with different band pass filters.

2. Fast Fourier Transform FFT to transform the recordings from the timedomain into the frequency domain. We can do this for the entirerecording (2A) or for shorter times by running windows over the record(2B). In the case of 2B, the window length should be at least a fewperiods of the investigated wave. This is the calculation of aspectrogram.

3. Selection of time periods (optional but recommend; only in case 2B):The spectrogram allows selection of time periods from the recording. Itis possible to select the quietest time period to reduce or eliminateundesirable influences due to transient signals (e.g. earthquakes orother anthropogenic noise). Selection of time period: There aredifferent methods for selecting data from the calculated spectrogram.Some examples: 1. A time period may be selected where the energy withina defined frequency window is minimal. 2. The amplitudes may be sortedwithin the spectrogram. This means that, for each frequency, theamplitudes are sorted from low to high. Afterwards, it is possible toselect the quietest time periods for each frequency and each time.Finally summing over a time period may be performed to stabilize thedata and to obtain only one amplitude spectrum which may be supplied tothe next processing step. 3. A further method for the selection ofuseful time periods can be performed in the time domain beforeperforming the FFT. Thereby the energy may be estimated within everytime window (or e.g. calculating the RMS value for such a short timewindow). Afterwards only the time periods with the lowest energy contentare used. 4. Another selection method is described by Groos and Ritter(2008; submitted to Geophysical Journal International) and the Diplomathesis by Joern Groos (2007). The time series is divided into periods ofdifferent noise classes.

4. As described hereinbefore, data acquisition may be performed invarious different ways to cover the surveyed area with sensors. 1. iffor every desired position there is a sensor, it is not necessary toperform a normalization. Instead, the same time periods are selected foreach sensor. In the case 2a, the permanent station (own sensor or publicsensor) may be used for performing a normalization. The spectra of allrecordings may be divided by the time equivalent amplitude spectrum ofthe recording of the permanent station. Before performing the division,it may be useful to smooth the spectra in advance to give stabilizedresults. Doing it in this way allows only the difference related to thegeology between the different locations to be seen. In case 2b, there isno permanent station for the entire survey, but there are “temporarilypermanent” sensors. In this case, it is possible to calculate thedifference/quotient between the second sensor and the first (the firstsensor is used for the normalisation of the second sensor). After movingthe first sensor to the position C for the next recording phase, thesecond sensor stays at its position and may be used for thenormalization as a permanent station. In this way, the change betweentwo sensors is always being estimated. Finally, the change between eachsensor and the first station may be calculated so as to provide anormalized map of changes to the first station. The results of anapplied normalization are comparable amplitude spectra for each sensorlocation.

5. Value Calculation: There are some possible methods to calculate afinal value for each location:

-   -   The simplest way is to use the amplitude value for a specific        frequency.    -   Calculation of an integral of the amplitude spectra over a        specific frequency window. This frequency window may be between        0.001 and 1 Hz. The width of the frequency window may be        variable and may depend on the data and the desired resolution.        The result is one single value for each location.

6. The following processing steps are only to visualize the results:

I. It is possible to put all the calculated amplitude spectra from thestep 4 together to obtain a 2D profile or a 3D cube of the subsurface.In this case, the frequency is used as the depth coordinate. To convertthe frequency into depth, it is recommend to compare the observedchanges in amplitudes with changes in the geological structure (ifavailable) or to do a rough estimate (e.g. depth=c/f*0.25; withc=propagation velocity and f=frequency) or to use every synchronisedrecording of two different stations and estimate a local dispersioncurve (representative of the path between the two sensors) by performinga cross-correlation followed by a frequency time analysis (e.g. Bensonet al. 2007). A further inversion of these dispersion curves allows asubsurface model of shear wave velocities to be estimated which may beused for a calculation of eigenfunctions to estimate for each frequencythe depth of the highest amplitude (Friedrich & Dalkomo 1995). Thisinformation may be used for a very precise depth conversion.

II. The calculated values of each location from the step 5 may beplotted as a map to visualize the results.

It is thus possible to create an image of the rough reservoir structure.Compared to active seismic, this technique is highly cost-efficient andenvironmentally friendly, but of lower resolution.

In embodiments where “quiet” time periods are selected from the records,it is possible to be more confident that the data are not influenced, orare at least less influenced, by transient signals or otheranthropogenic sources. Also, this selection allows data which were notrecorded simultaneously to be compared.

Some embodiments allow the horizontal component of Rayleigh waves andLove waves to be analyzed. This technique is thus not limited toanalysing only the vertical component of Rayleigh waves.

By covering the whole area with sensors, it is also possible to applyother methods of analyzing the data. For example, the same data may beanalyzed using ambient noise surface wave tomography.

In embodiments which provide local dispersion curves, more precise depthestimation may be provided. It is thus possible to provide improvedvertical resolution.

It is possible to produce a rough image of the subsurface with a veryinexpensive method and of relatively good resolution. This technique isindependent of the environment, so that it is possible to apply it onmountains, in deep sea, in arctic areas or in protected national parks,where is it is not possible or not allowed to use active seismic sources(especially in this frequency band). The technique may provideadditional information and may be used in combination with other methods(e.g. ambient noise tomography).

It is believed that a new exploration technique has been provided andmay be used in exploration for hydrocarbon reserves or for relatedfunctions, such as monitoring production in known hydrocarbonreservoirs. This technique is relatively inexpensive and it may bepossible to make use of already existing passive seismic measurements orrecordings.

The technique is of the passive type and may be used, it is believed,anywhere in the world. For example, the technique may be used offshoreor onshore or even in arctic regions, for example above ice sheets. Itmay be possible to provide three-dimensional information about thesubsurface structure and/or the technique may provide a directindication of the presence of hydrocarbons.

The invention claimed is:
 1. A method of conducting a survey to explorea subsurface region below a surface of the Earth, comprising: placing aplurality of sensors at respective sensor locations to obtain seismicdata by recording ambient seismic waves at said respective sensorlocations, moving at least some of said sensors during the survey to aplurality of different respective sensor locations in turn to form aplurality of different, successive combinations of sensor locations,each of said successive combinations of sensor locations including atleast one sensor location that is a same sensor location as in the justprevious combination of sensor locations, and making at each combinationof sensor locations at least partially simultaneous recordings with eachsensor to obtain seismic data, each said sensor being sensitive in afrequency range whose lower limit is greater than or equal to 0 Hz andwhose upper limit is less than or equal to 1 Hz; and processing avertical component of said data to obtain an amplitude of energy in afrequency band within said frequency range to obtain informationrelating to an interface in the region and to display an image of thesubsurface region.
 2. The method of claim 1, in which each of saidsensors is sensitive to two orthogonal horizontal said components andone vertical said component.
 3. The method of claim 1, comprisingrotating said components so that one thereof is aligned in a desireddirection.
 4. The method of claim 3, in which said desired direction isa direction of arrival of said waves.
 5. The method of claim 1, furthercomprising using a frequency obtained from said seismic data to estimatea subsurface depth at which the interface occurs in the region.
 6. Amethod of conducting a survey to explore a subsurface region below asurface of the Earth, comprising: placing a plurality of sensors atrespective sensor locations, each of which is moved in turn to aplurality of different sensor locations during the survey, to obtainseismic data by recording ambient seismic waves in a frequency rangewhose lower limit is greater than or equal to 0 Hz and whose upper limitis less than or equal to 1 Hz and the moving of each of said sensorsduring the survey forming a plurality of successive, differentcombinations of sensor locations, each of said successive combinationsof sensor locations including at least one sensor location that is asame sensor location as in the just previous combination of sensorlocations, and making at each combination of sensor locations at leastpartially simultaneous recordings with each sensor to obtain seismicdata, and processing a vertical component of said data to obtain anamplitude of energy in a frequency band within said frequency range toobtain information relating to an interface in the region and to displayan image of the subsurface region.
 7. The method of claim 6, in whichsaid plurality of sensors comprises first and second said sensors and inwhich said first and second sensors are moved alternately to providesaid different combinations.
 8. The method of claim 7, comprisingnormalizing said data provided by each of said first and second sensorsfollowing a change of position thereof with respect to said dataprovided by another of said first and second sensors.
 9. The method ofclaim 6, further comprising using a frequency obtained from said seismicdata to estimate a subsurface depth at which the interface occurs in theregion.
 10. A method of conducting a survey to explore a subsurfaceregion below a surface of the Earth, comprising: placing a plurality ofsensors at respective sensor locations, each sensor being successivelyplaced at a plurality of locations during the survey to obtain seismicdata by recording ambient seismic waves in a frequency range whose lowerlimit is greater than or equal to 0 Hz and whose upper limit is lessthan or equal to 1 Hz and the placing of each of said sensors during thesurvey forming a plurality of successive, different combinations of saidlocations in turn, each of said successive combinations of sensorlocations including at least one sensor location that is a same sensorlocation as in the just previous combination of sensor locations, andmaking at each combination of sensor locations at least partiallysimultaneous recordings with each sensor to obtain seismic data;selecting said data in at least one time sub-interval of each of atleast some of said recordings in accordance with a quality of said datain said sub-interval; and processing a vertical component of saidselected data to obtain an amplitude of energy in a frequency bandwithin said frequency range to obtain information relating to aninterface in the region and to display an image of the subsurfaceregion.
 11. The method of claim 10, in which said data are selected inaccordance with said energy contained in said sub-interval.
 12. Themethod of claim 11, in which said data are selected from saidsub-interval of each said recording containing a smallest amount of saidenergy.
 13. A method of conducting a survey to explore a subsurfaceregion below a surface of the Earth, comprising: placing a plurality ofsensors each at a plurality of sensor locations during the survey toobtain seismic data by recording ambient seismic waves in a frequencyrange whose lower limit is greater than or equal to 0 Hz and whose upperlimit is less than or equal to 1 Hz and moving each of said sensorsduring the survey to form a plurality of successive, differentcombinations of said sensor locations in turn, each of said successivecombinations of sensor locations including at least one sensor locationthat is a same sensor location as in the just previous combination ofsensor locations, and making at each combination of sensor locations atleast partially simultaneous recordings with each sensor to obtainseismic data; and processing a vertical component of said data to obtainan amplitude of energy in a frequency band within said frequency rangeto obtain information relating to an interface in the region, includingperforming cross-correlation of said data recorded simultaneously at atleast one pair of said locations, deriving wave dispersion data fromsaid cross-correlation, and inverting said dispersion data to locate adepth of highest wave amplitude between said locations of said at leastone pair, and using the obtained information to display an image of thesubsurface region.
 14. A method of conducting a survey to explore asubsurface region below a surface of the Earth, comprising: placing aplurality of sensors each at a plurality of different sensor locationsduring the survey to obtain simultaneously recorded seismic data byrecording ambient seismic waves in a frequency range whose lower limitis greater than or equal to 0 Hz and whose upper limit is less than orequal to 1 Hz and the moving of each of said sensors during the surveyforming a plurality of successive, different combinations of said sensorlocations in turn, each of said successive combinations of sensorlocations including at least one sensor location that is a same sensorlocation as in the just previous combination of sensor locations, andmaking at each combination of sensor locations at least partiallysimultaneous recordings with each sensor to obtain seismic data; andprocessing without normalizing a vertical component of said data toobtain an amplitude of energy in a frequency band within said frequencyrange to obtain information relating to an interface in the region andto display an image of the subsurface region.